I saw someone sloping a motorless QB, it flew really well, and in light lift. It wasn't very manueverable, so thermalling might be hard, but it has a great glide ration, espesially with a small, light motor/battery setup.

It seems that as a glider the Queen Bee is supposed to have a flying weight of around 1,4 kg. Can an owner of it confirm this?

I was thinking about a brushless motor with 1,5kg thrust which weights about 150g - do you think this would be a good choice for this bird?

I also have an question in addition to that (in particular for Queen Bee owners): How much should my battery back (assumed that it is close to the nose) weight in order to balance the Bee at its CoG, taking into consideration the 150g motor?

I think that with brushless + lipos you could get it down to less than that, IMO it would fly much better if you put a smaller power system on it. I think Mega 16/15/4 + 2s TP2100 + 7x5 apce would be a nice 'sailplane' power setup. Maybe up the battery to 3s +2500mah if you want more 'oomph'.